Anode target for high-voltage highvacuum uniform-field acceleration tube



y 3, 1951 R. J. VAN DE GRAAFF ETAL 2,559,526

. ANODE TARGET FOR HIGH-VOLTAGE HIGH-VACUUM UNIFORM-FIELD ACCELERATIONTUBE 2 Sheets-Sheet 1 Original Filed Sept. 18. 1945 ll? Iii 18 Jive/@211July 3, 1951 R J VAN DE GRAAFF ETAL ANODE T'AR'GET FOR HIGH-VOLTAGEHIGH-VACUUM UNIFORM-FIELD ACCELERATION TUBE Original Filed Sept. 18.1945 2 Sheets-Sheet 2 Patented July 3, 1951 UNITED STATES PATENT2,559,526 ANODE TARGET FOR HIGH-VOLTAGE men- VACUUM UNIFORM- TUBE FIELDACCELERATION Robert J. Van de Graafl, Belmont, and William WeberBuechner, Arlington, Mass., assignors to Research Corporation, notationof New York Original application Se New York, N. Y., a corptember 18,1945, Serial Divided and this application December 20, 1949, Serial No.133,972

3 Claims. (01. 313330) 1 This application is a division of ourco-pending application Ser. No. 617,036, filed September 18, 1945, nowPatent 2,517,260.

This invention relates to anode targets for high-voltage high-vacuumuniform-field acceleration tubes, and constitutes an entity or structurepatentable per se or apart from the remainder of the apparatus forgenerating an accurately focused beam of charged particles; eitherelectrons or ions, disclosed in our said copending application Ser. No.617,036, with which,

however, it is advantageously and preferably used. Our inventionprovides and constitutes an anode target which, when subjected tobombardment by a concentrated beam of such swift particles, melts inpart, but still remains mechanically strong and does not disappear as byevaporation, so that the concentrated beam of swift particles alwaysbombards the same material and does not bore through the target and soeventually bombard the target support.

In order fully to set forth the operation as well as the construction ofthe said anode target, we will describe sufiiciently the apparatus inwhich it is used and of which it forms a part.

The invention makes possible an improvement in high-voltage radiographyand is applicable to high-voltage vacuum tubes. The invention is usefulin such fields as nuclear physics, cancer therapy, radiography,high-voltage X-rays, the rectification of high-voltage currents, theproduction of cathode rays and the acceleration of electrons forhigh-voltage electric microscopes.

The high-voltage vacuum tube herein disclosed and in which our anodetarget invention is used and of whichit forms a part, is a constantpotential X-ray tube of the order of two million volts suitable foroperation either sealed oil or with a continuously operated vacuum pump.

Inasmuch as it is appropriate and desirable to disclose substantiallythe entire apparatus for a complete understanding of our invention andthe operation and advantages thereof, it is to be noted that suchapparatus comprises a highvoltage tube of insulating material havingelectrodes adapted to be connected to a suitable source of highpotential, such as a high voltage electrostatic generator. At one end ofthe tube, bein the upper end as shown in the drawings, is located meansproviding an emitting source, which means in the present disclosure is afilament having a plane emitting surface of relatively minute area. Aswill be subsequently described in detail, the wall of the tube compriseselectrode rings or ring-like disks or centrallyopen metallic diaphragmsarranged along the tube, spaced by insulation such as glass, andconnected in suitable manner to the corresponding electrodes in ahigh-voltage generator in such a way that the potential gradient downthe tube is uniform, and in such a way that equal steps in the voltagebetween successive" electrodes are provided. Thus, there is provided inthe tube a substantially uniform electrostatic field.

The focusing of the electron beam by the herein disclosed apparatus andby the use of a substantially uniform electrostatic field, is lesssensitive to variations in the potential applied to the various tubeelectrodes than is the case in tubes employing non-uniform electrostaticfields.

One advantage resulting from the use of a substantially uniformelectrostatic field, in combination with a magnetic lens in ahigh-Voltage vacuum tube, is that thereby extremely fine focusing isobtained of a high-speed electron beam of the order of two millionvolts. Moreover, it appears that the construction referred to, oneembodiment of which is herein disclosed, being simpler, is more reliablethan prior constructions. The employment of a substantially uniformelectrostatic field is ,materially associated with breaking up thevoltage along the tube into very small divisions, which is alsodesirable from the point of view of insulating very high voltage.

With a uniform electrostatic field substantially the entire crosssection of the high voltage vacuum tube can be used for the accelerationof the chargedparticles, whereasin a non-uniform electrostatic field, asheretofore generally employed, the region which is usable for focusingis usually very close to the axis and is most effective only for theparaxial rays. Thus, with the use of a uniform electrostatic field theelectrons may be suitably accelerated through a region whose crosssectional diameter is relatively large when compared with the length ofthe high the electrons is done in a uniform electric field, there ismade a full and direct use of the longitudinal component of the electricfield, with a minimum interplay or even presence of the transversecomponent of the electric field, a component in itself useless for theacceleration of the ions in the desired direction.

Heretofore in attempting to focus the electron beam, means have beenused constituting a relatively complicated guidance or compulsion. Itappears that, both in theory and in practice, if the electrons arepermitted to fall in or be accelerated by a simple uniform electrostaticfield, the result is more satisfactory than the result obtained withmore complicated means, the elements whereof require a certain definite,simultaneous adjustment relative to each other.

In some prior high voltage tubes a part only of the tube had asubstantially uniform electrostatic field, but in all such cases knownto us the part of the field of such tubes that are nonuniform incharacter was actually the part that was the most important of all asregards directing the motions of the charged particles. Thus, where inprior instances, a uniform electrostatic field was created in part of ahigh voltage tube, it was not primarily for the purpose of focusing abeam, but mainly to simplify other features of the construction. Aninstance thereof is shown in the patent to Trump, No. 2,182,185, abovereferred to.

In certain other tubes of the prior art the very beginning of the pathof the electrons or ions was not in a uniform field and was actuallysharply distorted, so that there resulted an initial spontaneousbreakdown creating a localized source of ionization by virtue of thefact that the electrostatic field was extremely non-uniform incharacter. Also in such instances in the prior art, tubes were made foroperation with impulses where the voltage was on for periods of theorder of only a few microseconds each, and in order to pass sufficientaverage currents they had to have high instantaneous currents.

The momentary breakdown in the tube afforded extremely highinstantaneous currents, so high that the accompanying space charge wouldtend to distort, during the moment of actual operation of the tube, theuniformity of the electric field in regions which had been uniform justprevious to the discharge.

In the apparatus herein disclosed, in order to provide a path for theelectrons or charged particles through the high voltage tube, preferablya two-inch diameter hole is cut out of or is otherwise provided at thecenter of each of the metallic ring-like electrode disks or diaphragmsprovided along the extent of the tube. Since such successive electrodering or diaphragm is more and more positive from the filament toward thetarget, the electrons or negatively charged particles are attracted downthe tube and strike the target with an energy corresponding to the fullgenerator voltage. The conditions are reversed when positive ions are tobe accelerated. In their passage down the tube, they tend to follow thelines of electric force, and in the high-voltage vacuum tube hereindisclosed, the lines of force are straight lines. Consequently when theelectrons or charged particles reach the bottom of -the tube and strikethe target, they are all traveling in parallel paths and all have thesame energy. Such a result could hardly be secured where an alternatingcurrent device, such as a transformer, is used for the voltage source,be-

cause in such case the electrons usually have all energies ranging fromsome indeterminate low value up to that corresponding to the peak of thealternating current wave. There is thus an essential difference betweendirect current equipment where all the electrons striking the targethave the full generator energy, and all alternating current equipmentwhere only a few of the electrons have the full rated peak voltage, theremainder being of lower energy.

The structure herein disclosed is equally well suited for theacceleration of either positive ions or negatively charged particles.This follows since the manner of construction and the use make the tubecompletely symmetric. Thus, it is possible to accelerate charges ineither direction through the tube without the necessity of having tochange the arrangement of potentials on the electrodes. The electronsare emitted at the negative end of the tube and are accelerated towardthe electron-collecting target, while at the same time positive ions areto be produced at the positive end of the tube and accelerated towardthe region of the cathode.

Other things being equal, the diameter of the beam of charged particlesafter passing through the tube is proportional to the size of the sourceof the charged particles. When the apparatus is used as an X-ray tubefor radiography, the definition in the radiograph depends criticallyupon the spot size, and hence it is very desirable that the effectiveportion of the filament be as small as possible. As more fully'set forthin the description of the drawings, the focused spot upon the target canbe smaller than 0.01 of an inch in diameter.

To obtain radiographs of thick sections having good definition, the sizeof the focal spot must be very small so that the X-rays will beemanating from a point source. Thick metallic sections of objectsrequiring on the order of two million volt X-rays present new geometricproblems making essential the use of such size of focal spot. As stated,the high voltage tube operates in conjunction with an electrostaticgenerator producing a potential of the order of two million volts. Theuse of such constant potential has been found necessary in order toobtain and to maintain the extremely fine focusing referred to and toprovide optimum conditions for heat dissipation at the focal point. Assubsequently set forth in detail, the target, upon which the electronbeam is focused and to which the present invention is particularlydirected, is a thick disk of gold used in association with high pressurewater cooling. The use of such a relatively thick target disk permitsoperation with the target spot in molten condition without, however,melting entirely through the disk. It becomes possible as a result tomake full use of the high intensity, sharply concentrated, electron beamand thus to obtain X-ray pictures of greatly improved quality.

The invention will be better understood in detail by reference to thefollowing description when taken in connection with the accompanyingillustration of one specific embodiment thereof, while the inventionwill be more particularly pointed out in the appended claims.

In the drawings:

Fig. l is a vertical or longitudinal, central, cross section of ahigh-voltage vacuum tube wherein the anode target herein claimed is usedand wherewith such anode target cooperates;

Fig. 2 is a transverse or cross section upon the line 2-2 of Fig. 1;

Fig. 3 is a detail invertical central section through the lower end ofthe filament and the surrounding guard ring;

Fig. 4 is a view similar to Fig. 3, but on a larger scale andrepresenting only a portion of the guard ring; and

Fig. is a broken-away detail in side elevation of a portion of thehigh-voltage vacuum tube shown in Fig. 1, with a diagrammatic indicationof the connections between the electrode rings of the tube andcorresponding electrodes of an electrostatic generator.

Referring to the drawings, there is shown a high-voltage vacuum tubeconsisting of a column of glass rings and of metal electrode rings orring-like diaphragms or disks suitably welded together in alternationthroughout the column, a part only of which is shown in a manner notherein necessary to disclose in detail. When the apparatus is used as anX-ray tube, the electron beam is controlled and compelled to strike thetarget at a point of exceedingly small diameter.

This configuration of electric field is also well suited for theacceleration and focusing of ion beams.

Fig. 1, the glass rings are respectively indicated at l, and the metalelectrode rings, centrally open *diaphragms or disks at 2. The saidmetal rings 2 or the like are electrode rings and lie accurately placedin planes perpendicular to the axis of the tube, and they are placed atequal distances apart, as, for example,vone-third of an inch in thepresent disclosure. represented as broken away because of the necessityof presenting a view of the complete tube in a single figure. Whileobviously the invention is 'not limited to any particular size orproportion of parts, it is pointed out that in the illustratedembodiment of the high-voltage tube the distance in the actual structurefrom the horizontal line 3' to the horizontal line 4 is fifty-seveninches, the diameter of the opening in each ring 2 is two inches, andthe outside diameter of the tube or column is three inches. As clearlyshown in Fig. 3, the outer edge of each of the metal electrode rings 2is substantially coterminous with the outer edge of the glass rings I.

In the simplified form .of high-voltage tube, represented in Fig. 1, thedistance from the line 3 to the top of the dome-like glass insulation isabout six inches. As stated, however, these dimensions may be varied asfound suitable, and the scope of the invention is in no wise restrictedby this recitation of dimensions.

In the plane of the top metal electrode ring 2, a metal disk 5 isprovided which substantially fills the opening inside said topmost metalring 2, which disk 5 is maintained at the same potential as the topmetal electrode 2. This insures that the electric field immediatelybelow the region of the disk 5 is uniform. The glass insula-.

In Fig. 1, the tube or column is tion which holds the metal disks 2 incorrect relative alignment and which consists of the glass rings I, mayhave on its inner surface an uncontrolled distribution of electriccharge which would tend to distort in a random and uncontrolled mannerthe uniformity of the electrostatic field within the main region of thetube. However, the disturbing charges'is reduced to a negligible degreeby the shielding effect of the metal rings 2, which extend influence ofthese inward from the glass wall composed of the glass rings I towardthe axis of the tube to a sufilcient extent to produce the desiredshielding. The fact that the gap between adjacent metal rings 2 isrelatively small, being in the present disclosure one-third of an inchless the thickness of one disk, the actual structure having the otherproportions above specified, makes it possible to obtain the desiredshielding efiect with only a relatively narrow region or portion of eachmetal ring extending inwardly beyond the inner surface of the glass wallcomposed of the multiplicity of glass rings I.

In the present disclosure the amount that each metal ring must projectinward from the glass wall of the tube must be approximately the same asthe length of the gap between next adjacent metal rings 2 all along theglass wall ofv the tube. Thus, the fact that in the present disclosurethe gap between the next adjacent metal rings 2 is small is in itselfadvantageous, inasmuch as it reduces the amount that each metal ring 2must extend inward beyond the inner surface of the glass wall.

In order to obtain a uniform electrostatic field, it is essential thatthe metal rings 2 be close together. The fact that they are placed closetogether makes it possible to insulate a high voltage per unit length ofthe tube.

Moreover, the fact that the metal rings 2 are close together makes itpossible to use more of the internal space in the tube for the beam ofcharged particles.

Certain metal rings 2 of the tube or column, which are indicated at 2ain Fig. 5, are connected to corresponding electrodes of the generatingapparatus which may take the form of a highvoltage electrostaticgenerator, as indicated in the diagrammatic part of Fig. 5 in such a waythat the voltage between the successive electrodes of the tube is thesame.

In Fig. 5 a few of the generator electrodes are represented at 2b, and aportion of the resistors at 20. As shown, every third electrode ring 2of the. tube is connected to a corresponding electrode of the generator,which generator electrodes are an inch apart. Each of the metalelectrode rings 2 in Fig. 5, as well as in Fig. 1, has its outer edgesubstantially coterminous with the outer edge of each of the glass ringsI.

As has been stated in the foregoing, the acceleration of an electronbeam in a'uniform field has many basic advantages as contrasted with themore usual methods of acceleration in strongly non-uniform electricfields. However,

it may be desirable while still using a substan-' tially uniformelectric field for acceleration to modify it or warp it slightly, forexample, in dealing with certain practical situations which would notarise in an entirely ideal case. In order toovercome the spreadingeffect, due to the space charge of a positive ion beam, it might bedesirable to have the top part of the accelerating electric fieldslightly converging. This condition could be realized simply by havingthe voltage difference from electrode to electrode constantin the lowerand middle portion of the tube, but with this voltage differenceslightly decreasing as the very top of the tube is approached.

Referring to the use of the apparatus as an X-ray tube, the filament ofthe tube from which emanates the electron beam is indicated at I0 inFig. 1, and is shown in detail in Figs. 3 and 4. The said filament iscomposed of tungsten, and is of a hairpin type. It has the apex of thebend ground ofi, as indicated at II in Figs. 3 and 4, in order toprovide a plane emitting surface II of relatively minute area. Thediameter of the filament in the unreduced portion thereof is 0.010, andat the ground-off portion of the apex of the bend it is desirably lessthan one-half such thickness, thereby insuring an intense heat at saidground-off portion when the apparatus is in use, being the planeemitting surface of the electrons. The cross section of the filamentbeing the least at the ground-01f portion, the resistance is thegreatest at that area.

The filament I has placed in conjunction therewith and encircling thesame, a guard ring l2, shown enlarged and in part in Fig. 4, which has aplane lower surface lying exactly in the same plane as the emittingplane of the filament. The said guard ring l2 has therein a centralthrough-opening l2a, which is approximately 0.040 of an inch in diameterand within which the apex of the bend, constituting the plane emittingsurface I I, is symmetrically positioned.

The filament and the surrounding guard ring are usually maintained atapproximately the same potential. However, by making the potential ofthe guard ring substantially more negative than that of the filament,the grid action of the guard ring can be used to reduce, or evenentirely out off, the electron stream. For this purpose there are shownin Fig. 4 wires la and lib leading respectively from the filament l0 andfrom the guard ring l2 to the positive and negative sides of a batteryB. Also the over-all focusing properties of the tube as a whole may beaffected by providing relatively small voltage differences between thefilament and the surrounding guard ring. Thus, although the filament andthe guard ring have been generally operated at the same potential, thereare some occasions when it is desirable to operate the filament andguard ring at somewhat different potentials.

By reason of the plane emitting surface ll of the filament l0 and of theuniform field within the tube column, the beam of electrons proceeds ina substantially straight line along the tube or column from the point ofemission, as indicated at l3, resulting in a beam whose cross section inthe region near the top of the tube corresponds closely to the size andshape of the emitting plane, and wherein the energy of the individualcharged particles is substantially identical. Such a beam may readily befocused by a relatively weak magnetic field on an extremely concentratedspot, as by an electric magnet H, the arrangement constituting amagnetic lens, the magnetic lines of force whereof are indicated at Ma.

Where the apparatus is used for generating X-rays, as for high voltageradiography, the electron beam is focused on a target which is a thickmetal disk 15 of gold, used in association with a high pressure watercooling jacket, indicated at Hi, and provided with a water inlet I1 andwater outlet la. The target I5 is a gold disk one-quarter of an inch inthickness.

With the usual construction for X-ray targets, a high voltage beam ofelectrons of great concentration would melt locally the target employedin such construction, and thus cause leakage of the cooling water in thevacuum of the X-ray tube, or cause cracking of the tungsten target andimpair its usefulness. This would prevent further use of any such deviceuntil repaired. However, with a thick target of material such as gold,which has a high melting point and high heat conductivity, the moltenregion is small, and since it does not extend entirely through thetarget, 'no leak is caused. The surface tension of the liquid gold tendsto keep the gold from flowing away. It is observed in practice that the8 vapor pressure of the liquid gold is so low, under the operatingconditions that the thinning of the target due to evaporation isnegligible.

Although tungsten has generally been used as a standard material fortargets, experience with the gold targets herein disclosed indicatescertain advantages. Gold has a high heat conductivity and also chemicaland physical properties such that it can be repeatedly melted andallowed to freeze without appreciable oxidation or change in physicalstructure.

Since the efficiency of X-ray production rises rapidly with the atomicnumber of the target material, x-ray tube anodes are commonly made ofthe heavy metals. Tungsten has been the traditional material for thispurpose, primarily because of its very high melting point. In the usuallow-voltage tube, the target is quite often allowed to operate at whiteheat, and, when cooling is necessary, the target is usually embedded ina massive disk of copper that may be cooled either by water or by an airblast. In such low-voltage tubes, the penetration of the electrons intothe target material is so slight that the energy is deliveredessentially to the target surface, and radiation from the surface playsa very large part in dissipating the heat energy so generated.

Such tungsten targets may also work satisfactorily for tubes operatingin the million-volt range if the focal-spot size is large enough so thatthe power density on the target is not excessive. However, acceleratingtubes of the uniform-field type can deliver an electron beam soconcentrated that the power density on the target is suflicient to meltany material. With these high-energy densities, it is essential that theheat be transferred as rapidly as possible from the focal point to thesurrounding unbombarded target material. This requires a target materialhaving a high conductivity rather than a high melting point. Gold hastwice the thermal conductivity of tungsten, and, in addition, has ahigher atomic number. Moreover, its other physical properties, such asease of soldering, malleability, and so forth, give it many advantagesover tungsten for this particular application,

Unfortunately, those materials that are most suitable for the efficientproduction of X-rays are also the best X-ray absorbers. For low-voltagetubes, the X rays produced are not sufficiently penetrating to passthrough the target structure; hence, the radiation is commonly broughtout through the side of the X-ray tube. This is not a seriouslimitation, since, at these low voltages, the X-radiation has a spatialdistribution that is essentially symmetric about the position where theelectron stream strikes the target. This is not the case forhigh-voltage X-ray tubes, since here the radiation is produced primarilyin the direction of the electron beam. For this reason and also becauseit is generally more convenient from the point of view of construction,the radiation from such high-voltage tubes is allowed to pass directlythrough the target structure. To reduce the absorption of the radiationin the target, it has been customary to make the target as thin aspossible. There are numerous references in the literature to thintargets which are cooled on the side away from the vacuum by a stream ofwater or air. Such thin targets have been found to be unsatisfactorywhen used with concentrated electron beams, such as those produced bythe uniform-field accelerating tube herein disclosed. The high currentdensities employed on the targets herein disclosed are sufficient tomelt the material, with the result that the pressure of the coolingmedium was sufficient to force a hole through the target, thuspermitting the cooling medium to enter the highvacuum tube. In anattempt to prevent this, previous workers had made the targets eventhinner in an attempt to bring the cooling medium closer to the regionwhere the heat was being produced. Such attempts were not successful,and our investigations and experimentsindicated that the best hope ofsuccess was to make the targets so thick that even if local meltingoccurred in the region of the focal spot. there was still sufii'cientmetal between this focal region and the cooling medium so as to preventpuncture. At first sight. it would appear that this additional targetmaterial would involve a'serious reduction in the beam intensity, butour work on the problem of the eflicient utilization of this radiationin the problems of radiography and therapy showed that the additionalfiltration provided by the thick targets was actually beneficial. andexperiments have shown that, were it not for the filtration provided bythe thick target, it would be necessary to put additional absorbingmaterial in the path of the radiation proceeding from the tube.

We have in accordance with our invention provided in a high-voltagehigh-vacuum tube for generating an [accurately focused beam of chargedparticles 'of great concentration upon a minute area, a thick anodetarget composed wholly of a metal having a high atomic number, the saidtarget, being sufiiciently thick so that even though under the action ofthe highly concentrated beam of charged particles impinging on it. thetarget material becomes molten at the point of impact, sufficient solidtarget material still remains surrounding the molten region to preventmechanical failure, and to which the molten material adheres by reasonof surface tension. The said anode target is too thick to permit thepassage of an accurately focused beam of charged particles therethrough,it having a thickness on the order of one-quarter of an inch, and

I hence is not of a thickness sufficient to prevent the passage ofhigh-energy X-rays. The said thick, high atomic'number, metal-anodetarget under the action consequent upon the passage of the chargedparticles therethrough becomes molten at the said minute point ofimpact, but the surface of the said metal-anode target there retains itsposition by reason of the high surface tension on the high atomic numbermetal while molten and because of the said thickness of said metal-anodetarget. Thus the said metal-anode target is prevented from meltingthrough under the impact of said focused beam of charged particles.

As already stated, we construct the said anode target of gold.

The present invention comprehends a highvoltage vacuum tube adapted tothe acceleration and focusing of charged particles, and in the case ofelectrons this beam is extremely concentrated. The disclosure includescharged particle accelerating means providing a uniform acceleratingfield. thus reducing to a minimum the dispersion of the chargedparticles throughout their travel. Therefore, a large number ofaccelerating sections are provided, the number used in present of saidelectrodes of which each such group is In fact, investigations composedis directly electrically connected to a correponding electrode of ahigh-voltage electrostatic generator, so that the voltage between thesuccessive disks of the tube isthe same.

Having thus described one illustrative embodiment of the invention, itis to be understood that although specific terms are employed, they areused in a generic and descriptive sense and not for purposes oflimitation, the scope of the invention being set forth in the followingclaims. We claim: 1. In a high-voltage high-vacuum tube for generatingan accurately focused beam of charged particles of great concentrationupon a minute area on the order of 0.01 of an inch in diameter, a thickanode target composed wholly of a metal having a high atomic-number,said anode target being on theorder of one-quarter of an inch thick sothat, even though under the action of the highly concentrated beam ofcharged particles impinging on it, the anode target material becomesmolten at the point of impact, sufficient solid target material stillremains surrounding the-molten region to prevent mechanical failure, andto which the molten material adheres by reason of surface tension, thesaid anode target being thus .too thick to permit the passage of anaccurately focused beam of charged particles therethrough, but not toothick to prevent the passage of X-rays therethrough.

2. For the production of high-voltage radiographs of very high quality,to be taken at relatively high speed through heavy objects,.a highvacuumacceleration tube in association with a high-voltage generator of theorder of a million or more volts for generating an accurately. focusedbeam of electrons upon a minute area approximately of the order of 0.01inch in diameter, such beam having a very greatpower density throughoutits cross-sectional area, the said acceleration tube having a targetanode composed of gold and having athickness on the. order ofone-quarter of an inch, and having a water-cooling jacket, the saidtarget being usable at temperatures farabove the melting pointlof gold,

said target material dfgOld becoming molten at the point of impact; butremaining sufficiently solid surrounding the molten region to preventmechanical failure, such as through penetration, the said gold anodetarget being therefore too thick to prevent the passage of theaccurately focused beam of charged particles, but not of a thickness toprevent the passage of high-voltage X-rays.

3. For the production of high-voltage radiographs of very high quality,to be taken at great L ticles upon a minute area on the order of 0.01

of an inch in diameter as a minimumsuch beam practice for two millionvolts'being approximately 180, thereby providing uniform acceleratingsteps of 12,000 volts each. Thus, in such embodiment having a verygreatpower density throughout its cross-sectional area, the said accelerationtube having a target anode composed of gold and having a thickness onthe order of one-quarter of an inch, and having a high-pressurewater-cooling jacket, the said target being usable at temperatures farabove the melting point of gold, said target material of gold becomingmolten at the point of impact, but remaining sufliciently solidsurrounding the molten region to prevent mechanical failure such asthrough penetration, the said gold anode target being therefore toothick to prevent the passage of the accurately focused beam of chargedparticles, but not of a thickness to prevent the passage of high-voltageX-rays.

ROBERT J. VAN m: GRAAFF.

WILLIAM WEBER BUECHNER.

REFERENCES crrEn The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS Number

